1. Field of the Invention
This invention relates to the purification of water, and more particularly concerns compositions for the selective removal of trace levels of dissolved metals from water.
2. Description of the Prior Art
Undesirable pollutant species such as dissolved toxic metals can occur regularly or periodically in natural bodies of water such as lakes, rivers, streams, groundwater and stormwater, and in industrial waters such as landfill leachate, municipal sewage systems and wastewater discharges. The toxic metals are primarily multivalent “heavy” metals, and are usually present at “trace” concentrations below about 10 parts per million (ppm) amidst a vastly higher concentration of commonly abundant, innocuous species such as sodium, potassium, calcium, magnesium and aluminum.
A well known technique for removing such trace toxic metals in dissolved ionic form is to pass the polluted water through a stationary bed containing a particulate substance capable of selectively absorbing the toxic metals without absorbing the commonly abundant species. Synthetic polymers having the desired selective absorption property have been available in bead form made by catalyzed “addition” type suspension polymerization of a liquid styrene/divinylbenzene prepolymer mixture. The resultant spherical beads generally have a size in the range of 16 to 50 mesh (U.S. Sieve Series), or 1.2 mm to 0.3 mm, respectively. The beads are subsequently subjected to chemical treatment which imparts chelation or complexation functionality. The resultant product generally has a metal-absorbing capacity of about 1 milliequivalent/dry gram of absorbent. For example, if saturated with Cu++, one dry gram of the absorbent would contain about 32 milligrams of copper, namely a 3.2% content of copper.
When employed as a stationary bed in a vertically oriented column through which water is caused to flow, said bead products produce a measurable impedance to the flow of water. The actual impedance, measured as psi/foot of bed depth, is dependent upon: a) flow rate, measured as gals/min./ft2 of bed cross section, b) average bead diameter, and c) temperature. In most applications, pumping at pressures up to about 30 psi, is required for effective operation of beds of spherical ion exchange beads. By way of comparison, beds of absorbents employed as “passive” or “in-situ” installations which depend upon hydrostatic pressure alone to propel the water, must be able to function with less than about 12 feet of hydrostatic pressure, or 5 psi.
When the water being remediated contains suspended matter such as soil particles, the suspended matter usually becomes filtered out within a bed of ion-exchange beads. Such action causes unacceptably high increase in the impedance to flow. Accordingly, the water to be treated must be pre-filtered to a total suspended solids (TSS) content below about 100 ppm. In fact, still other water pretreatments are commonly employed to protect the beads and thereby permit their re-use for economic considerations.
Other important factors concerning the use of water-treating absorbents are the speed and efficiency of operation. Bead form ion exchange absorbents generally have a kinetic performance such that a contact time of at least 2 minutes is required to produce at least 80% removal of the sought heavy metal ions. In applications such as the treatment of stormwater and other large flows, faster kinetics are required, generally at least 80+% removal within one minute of contact time.
Other synthetic polymers having selective metal-absorbing properties have been produced as granules of irregular size and shape, having been formed by “condensation” polymerization, namely the heating of an aqueous prepolymer solution. Examples of such non-bead form polymers for the selective absorption of dissolved metals are disclosed in U.S. Pat. Nos. 3,580,891 and 3,715,339, the disclosures of said patents being incorporated herein by reference. A classic example of a “condensation” type polymer is Nylon 6/6 produced by the DuPont Company of Wilmington, Del., wherein an aqueous prepolymer solution of hexamethylenediamine and adipic acid monomers is heated to drive off the water of solution and further heated to split out a molecule of water between interactive monomers.
Although certain granular polymers offer advantages over the bead form polymers produced by catalyzed suspension polymerization, the granular geometry has been found to produce unacceptably high impedance to the passage of water undergoing treatment. In attempts to overcome the problem of high impedance to flow, it has been sought to incorporate otherwise useful polymers onto or within large sized or porous carrier substrates. For example, the use of an open celled cellulose sponge of large monolithic shape as a carrier substrate is disclosed in U.S. Pat. No. 4,332,916. The use of small pieces of polymer-impregnated cellulosic sponge to form an absorption bed within a column for the selective removal of trace levels of heavy metals is disclosed in U.S. Pat. No. 5,064,540. Said patents are incorporated herein by reference. In fact, the small piece sponge product of U.S. Pat. No. 5,064,540 was the subject of a U.S. EPA Superfund Evaluation in 1994, reported as EPA/540/R-94/522, and incorporated herein by reference.
Other previously disclosed absorbents which either possess natural affinity for heavy metal ions or function as carrier substrates for appropriate polymers include peanut shells, soybean hulls, corn husks, tobacco stems, leaf compost, and minerals. Several shortcomings have been encountered with such previously disclosed absorbents, including: saturation capacities below 2%; slow kinetics; compaction during use; and disposal issues.
It is accordingly an object of the present invention to provide a water-treating product having selective affinity for dissolved heavy metal ions and presenting low impedance to the flow of water.
It is a further object of this invention to provide a water-treating product as in the foregoing object having a high saturation capacity and fast kinetics of operation.
It is another object of the present invention to provide a product of the aforesaid nature which includes a polymer derived from the thermal treatment of monomer components of an aqueous prepolymer solution.
It is an additional object of this invention to provide a water-treating product of the aforesaid nature which is relatively insensitive to suspended matter, and can be disposed in an environmentally acceptable manner.
It is a still further object of the present invention to provide a product of the aforesaid nature which is of sufficiently low cost to justify disposal following one time use.
These objects and other objects and advantages of the invention will be apparent from the following description.